In my experience, glass coverslips could be plasma oxidized and then could be coated with 0.2% gelatin. There is no need of glutaraldehyde crosslinking. It would allow proper cell adhesion onto the glass.
However, while cellular intrinsic response on any 2 materials is unlikely to be comparable. So, if you are targeting molecular studies, the cells on gelatin could show slightly different phenotype than on PLL. This is particularly due to the differences in cell adhesion motifs in both the materials.